Microfluidic device for inducing separations by freezing and associated method

ABSTRACT

A microfluidic device is provided for inducing the separation of constituent elements from a microfluidic sample by introducing phase changes in the microfluidic sample while contained in a microfluidic channel in the device. At least a portion of the microfluidic sample is frozen to cause fractional exclusion of the constituent element from the frozen portion of the microfluidic sample. Different portions of the microfluidic sample may be frozen in different sectors and at different times in order to cause movement in a desired direction of the separated constituent element. Portions of the microfluidic sample may be frozen in a sequential order of adjacent sectors within the microfluidic channel in order to cause sequential movement of the excluded constituent element toward one portion of the microfluidic channel. The frozen portion of the microfluidic sample is then thawed, wherein the separated constituent element remains substantially separated from the thawed, purified microfluidic sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 11/690,115, filed Mar. 22, 2007, now U.S. Pat. No.8,642,353, which is a continuation-in-part of U.S. patent applicationSer. No. 10/877,691, filed on Jun. 24, 2004, now U.S. Pat. No.7,757,717, which is a continuation-in part of U.S. patent applicationSer. No. 10/843,515, filed on May 10, 2004, now U.S. Pat. No. 7,694,694,the entire contents of each of which are incorporated by referenceherein.

TECHNICAL FIELD

This disclosure relates generally to the field of microfluidic devicesand, more particularly, to the separation and purification ofmicrofluidic samples.

BACKGROUND

Microscopic mechanical systems have been evolving for use in devices forsensing acceleration, pressure, and chemical composition, and have alsobeen used as actuators, such as moving mirrors, shutters, andaerodynamic control surfaces. More particularly, micromechanical systemshave been proposed for use in fluid control, such as in medicalpharmaceuticals, bearing lubricators and miniature space systems. Manytypes of fluid flow control systems require the use of pumps and valves.Developments in miniaturization and large-scale integration in fluidicshave led to the concept of creating microfluidic devices on amicroscopic scale.

One category of functions essential to many microfluidic analysisprocesses involves separations, and includes such functions asconcentration, separation, and purification. These functions are basedon the need to increase or decrease the concentration of a soluterelative to a solvent, or suspended particulates relative to a carrier.In macroscopic systems, methods employed for this function includefiltration, evaporation, and chromatography, none of which, for variousreasons, are easily implemented in microfluidic systems. Thus far,researchers experimenting with freezing as a method of separation on themacroscopic scale have met with limited success.

Freeze concentration is based on the phenomenon of exclusion of solutemolecules or particulates during crystallization of a solvent. Forexample, freezing a sample of salt water will produce a number ofcrystals of relatively pure water separated by regions of brine (eithersolid or liquid, depending of the final temperature of the sample).There are a number of difficulties associated with actually implementingfreeze concentration techniques. Once the sample is frozen, it is notpossible to separate the regions of relatively high and lowconcentrations of the solute because they are thoroughly interlocked,where this results from the difficulty in providing precise control ofthe freezing process.

A key problem is that an advancing ice front will push any solute aheadof it, causing a local buildup of solute concentration immediatelyadjacent to the ice front that will normally dissipate only throughdiffusion, which is an inherently slow process. As the soluteconcentration gets higher, the freezing point of the liquid isdecreased. The temperature of the ice thus has to get well below thenormal freezing point of the liquid in order to induce further growth ofthe ice. The proximity of the sub-cooled ice and the relatively warmerliquid beyond the region of elevated solute concentration sets up athermal gradient in the liquid, where the temperature of the liquidimmediately adjacent to the ice is below the normal freezing point ofwater. If the solute concentration gradient is sufficiently steepcompared to the thermal gradient, then the gradient in the localfreezing point of the fluid will be steeper than the thermal gradient inthe fluid. Under these conditions, the planar ice front becomesunstable, leading to a dendritic growth process in which tree-like solidstructures form. The dendrites trap regions of high soluteconcentration. If the temperature gradient is steep enough relative tothe concentration gradient, it may also be possible for freezing tonucleate in the fluid at a point some distance away from the ice frontand beyond the region of high solute concentration, again trappingsolute between layers of ice. In either case, the end result is a randomcollection of ice crystals trapping bands of high solute concentrationbetween adjacent ice layers. Because the freezing occurs irregularly,the bands of high solute concentration are irregular in shape, andcannot be easily separated from the relatively pure ice crystals.

Due to these difficulties associated with freeze separation on themacroscopic scale and to the lack of experience in providing propertemperature controls on the microscopic scale, freeze separation has notpreviously been considered possible for microfluidic devices. There is aneed for improved methods for achieving separations in microfluidicdevices.

SUMMARY

According to a feature of the disclosure, a microfluidic device andmethod are provided for inducing the separation of constituent elementsfrom a microfluidic sample by inducing phase changes in the microfluidicsample. In one aspect, the microfluidic sample is introduced into amicrofluidic channel in a microfluidic device, and at least a portion ofthe microfluidic sample is frozen to cause fractional exclusion of theconstituent element from the frozen portion of the microfluidic sample.In another aspect, portions of the microfluidic sample are frozen indifferent sectors within the microfluidic channel at different times inorder to cause movement in a desired direction of the constituentelement being separated from the frozen portions of the microfluidicsample. In another aspect, the portions of the microfluidic sample arefrozen in a sequential order of adjacent sectors within the microfluidicchannel in order to cause sequential movement of the excludedconstituent element toward one portion of the microfluidic channel. Thefrozen portion of the microfluidic sample may then be thawed, whereinthe separated constituent element remains substantially separated fromthe thawed and purified microfluidic sample.

In accordance with another feature, the microfluidic freeze separationdevice induces separations in a fluid containing solutes and/orparticulates that are confined within a microfluidic channel.Separations are inducted by triggering the formation of an ice crystalon one side of the microfluidic channel and growing the ice crystalacross the channel by controlling the temperature of the channel,thereby fractionally excluding and displacing the solutes and/orparticulates. By confining the process to a microfluidic channel in thedevice, several enhancements become possible that are not possible onmacroscopic scales, including precise temperature control, repeatingcycling to enhance separation, and cross-flow devices in which the fluidmoves in one direction while waves of freezing and melting move acrossthe channel in a direction perpendicular to the direction of fluid flow.

According to another feature, a microfluidic device and method areprovided for mixing fluids on a microfluidic scale in a microfluidicdevice. A plurality of fluids are introduced into a microfluidic channelin a microfluidic device. The fluids are then subjected to cyclicalfreezing and melting in different sectors of the microfluidic channel toachieve mixing of the fluids together.

DRAWINGS

The above-mentioned features and objects of the present disclosure willbecome more apparent with reference to the following description takenin conjunction with the accompanying drawings wherein like referencenumerals denote like elements and in which:

FIGS. 1A-1I are partial cross-sectional top views of a microfluidicfreeze separation device in accordance with one embodiment of thepresent disclosure.

FIG. 2 is a partial cross-sectional top view of a microfluidic freezeseparation device in accordance with one embodiment of the presentdisclosure.

FIGS. 3-5 are cross-sectional side views of various embodiments of themicrofluidic freeze separation device taken generally along linesIII-III of FIG. 2.

FIG. 6-9 are partial cross-sectional top views of a microfluidic freezeseparation device in accordance with various embodiments of the presentdisclosure.

FIGS. 10-12 are partial cross-sectional top views of a microfluidicfreeze mixing device in accordance with various embodiments of thepresent disclosure.

FIG. 13 is a graphical illustration of a zone of frozen microfluidicsample over time in accordance with one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

In the following description, numerous embodiments are set forth inorder to provide a thorough understanding of the invention. It will beapparent, however, to one skilled in the art, that these and otherembodiments may be practiced without these specific details. In otherinstances, well-known features have not been described in detail inorder not to obscure the invention.

For purposes of this description, a “microfluidic” device has one ormore channels or chambers with at least one dimension less than 1 mm.

There are many promising applications for a microfluidic freezeseparation device formed in accordance with the present disclosure,including but not limited to water quality monitoring, contaminationinvestigations, and in microbiological sample and analysis systems. Forexample, microfluidics may enable inexpensive, field-deployable systemsfor water quality monitoring or for biological sampling. Such devicescould potentially detect the presence of contaminants, pathogens, ortoxins in very small samples. Microfluidic handling processes involvedin these applications include valving, storage, pumping, metering, andthermal cycling. The Peltier-actuated microvalve and its derivatives,also developed by the present inventor, have assisted in making suchapplications possible.

Examples of microfluidic valve devices are described in detail in thefollowing patents and patent applications: U.S. patent application Ser.No. 11/190,312, entitled “Fast Acting Valve Apparatuses” filed on Jul.26, 2005, U.S. patent application Ser. No. 11/150,551, entitled“Electro-hydraulic Valve Apparatuses” filed on Jun. 9, 2005, U.S. patentapplication Ser. No. 10/877,691 entitled “Microfluidic Devices WithSeparable Actuation and Fluid-Bearing Modules” filed on Jun. 24, 2004,U.S. patent application Ser. No. 10/877,602 entitled “Microfluidic ValveApparatuses With Separable Actuation and Fluid-Bearing Modules” filed onJun. 24, 2004, U.S. patent application Ser. No. 10/843,515 entitled“Phase-Change Valve Apparatuses” filed on May 10, 2004, U.S. Pat. No.6,007,302 entitled “Mechanical valve having n-type and p-typethermoelectric elements for heating and cooling a fluid between an inletand an outlet in a fluid pump” issued on Dec. 28, 1999, and U.S. Pat.No. 5,975,856 entitled “Method of pumping a fluid through amicromechanical valve having N-type and P-type thermoelectric elementsfor heating and cooling a fluid between an inlet and an outlet” issuedon Nov. 2, 1999, the contents of which of all of the above-listedpatents and patent applications are incorporated herein by reference intheir entirety.

Referring now to FIGS. 1A-1G, cross-sectional top views of amicrofluidic freeze separation device 100 formed in accordance with oneembodiment are illustrated. The microfluidic device 100 includes amicrofluidic channel or chamber 102 for holding a microfluidic sample104 while separations of at least one constituent element 106 from amicrofluidic sample 104 are induced by introducing phase changes in themicrofluidic sample 104. In one aspect, the microfluidic sample 104 isintroduced into a microfluidic channel 102 in the microfluidic device100, and at least a portion of the microfluidic sample 104 is frozen tocause fractional exclusion of the constituent element 106 from thefrozen portion of the microfluidic sample 104.

In accordance with one embodiment, the microfluidic sample 104 will bedescribed as a solution containing water as a solvent having suspendedparticles or a solute, because water-based fluids are common in scienceand industry. However, it is the intention of the present inventor thatthe microfluidic device 100 can be similarly utilized to separatesolutions having non-water solvents. Further, the solubility of a soluteis generally but not always higher in a liquid phase than in a solidphase, and the microfluidic sample 104 will be described in severalembodiments as including a solute having a higher solubility in itsliquid phase. However, it is the intention of the present inventor thatthe microfluidic device 100 can similarly be utilized to separatesolutions in which the solubility of the solute is lower in a liquidphase than in a solid phase. The microfluidic device 100 achieves themicrofluidic separation process by inducing a phase change in at least aportion of the microfluidic sample 104, such as by cooling a carrierfluid or solvent to cause the formation of a solid phase that excludeseither suspended particles or a solute.

In one embodiment, the microfluidic channel 102 includes an inlet 106and at least one outlet 108. In one aspect, the dimensions of themicrofluidic channel 102 may be approximately 1-3 mm wide, 50 micronsdeep, and 25 mm long. The microfluidic sample 104 containing a soluteand/or suspended particles is introduced through the inlet 106 andthrough an inlet valve 110 into the microfluidic channel 102 until themicrofluidic sample 104 fills the microfluidic channel 102 to adesirable level, such as to a point adjacent to the outlet(s) 108, asillustrated in FIG. 1C. In one embodiment, the inlet valve 110 is amicrofluidic valve, such as a Peltier-actuated microvalve incorporatedby reference hereinabove.

After the microfluidic sample 104 is introduced into the microfluidicchannel 102, the inlet valve 110 is closed, as illustrated in FIG. 1C,by cooling the microfluidic sample 104 to form an ice plug 112 in themicrofluidic sample 104 adjacent to the inlet valve 110. Themicrofluidic channel 102 is then cooled asymmetrically such that atemperature gradient is created across the width of the channel suchthat the bottom edge 114 is colder than the top edge 116 of themicrofluidic channel 102. When the microfluidic channel 102 becomes coldenough, the microfluidic sample 104 freezes and an ice crystal 118begins to form along the bottom edge 114, as illustrated in FIG. 1C. Asthe frozen portion 118 grows, particulates, solutes or other constituentelements 120 are excluded from the frozen portion 118. As themicrofluidic channel 102 is further cooled, the frozen portion 118 growsacross the microfluidic channel 102 from the bottom edge 114 to the topedge 116, thereby transporting the excluded or separated constituentelement 120 to the top edge 116 of the microfluidic channel 102, asillustrated in FIG. 1D.

After the frozen portion 118 has grown across the majority (or possiblyall) of the microfluidic channel 102, the constituent element(s) 120(e.g., solutes and/or particulates) are concentrated in a narrow band ina sector 122 along the top edge 116 of the microfluidic channel 102. Themicrofluidic device 100 may then warm the frozen portion 118 to create apurified, melted microfluidic sample 124 in the microfluidic channel102, as illustrated in FIG. 1E. Melting of the frozen portion 118 doesnot directly cause any transport of the excluded constituent element(s)120, as the excluded constituent element(s) 120 will tend to remainalong the top sector 122 of the microfluidic channel 102. Diffusivetransport, which would tend to spread any particulates or solutes evenlyacross the width of the channel, is relatively slow and substantiallydoes not occur on the relevant time scales.

After the frozen portion 118 has melted, the flow of the microfluidicsample 104 through the microfluidic channel 102 is resumed by openingthe inlet valve 110. In one embodiment, a first outlet 108 a is providedfor the excluded constituent element(s) 120 while a second outlet 108 bis provided for the melted microfluidic sample 124, as illustrated inFIG. 1F. Because flow on the microfluidic size scale is strictlylaminar, the fluid containing the concentrated excluded constituentelement(s) 120 will travel out of the adjacent first outlet 108 a whilethe purified, melted microfluidic fluid 124 depleted of solute and/orparticulates will travel out of the adjacent second outlet 108 b.

In another embodiment, in order to provide further protection againstdiffusive transport of the excluded constituent element(s) 120 acrossthe microfluidic channel 102, the relatively pure solvent that remainsin the frozen portion 118 of the microfluidic sample 104 shown in FIG.1D may be left in the solid frozen state while the concentrated excludedconstituent element(s) 120 are pumped into the adjacent first outlet 108a, as illustrated in FIG. 1G. The portion 126 of the microfluidic sample104 in the upper sector 122 of the microfluidic channel 102 may then befrozen while the frozen portion 118 is melted, and the melted puresolvent 128 may then be pumped into the adjacent second outlet 108 b, asillustrated in FIG. 1H. Finally, the frozen portion 126 in the uppersector 122 of the microfluidic channel 102 is melted, as illustrated inFIG. 1I, and the microfluidic device 100 is then capable of beginningthe process again to induce separations in the microfluidic sample 104in the microfluidic channel 102.

Referring now to FIG. 2, a top view of one embodiment of a microfluidicfreeze separation device 100 is illustrated. The device 100 includes aninlet 106 and an inlet valve 110 on one side of the microfluidic channel102 and a pair of outlets 108 a, 108 b on the opposite side of themicrofluidic channel 102 from the inlet 106. An actuation module 130 ispositioned adjacent to at least one face of the microfluidic channel102, such as upper face 132 or lower face 134 (or possibly both faces132, 134 as illustrated in FIG. 3), so as to operatively interface withthe microfluidic channel 102 to introduce a phase change in themicrofluidic sample 104. In one embodiment, the actuation moduleincludes an array of actuation elements 136. In one embodiment, theactuation elements 136 may comprise an array of parallel linear thermalcontrol elements (TCEs 136), e.g., heaters and/or coolers, aligned in adirection parallel to the long axis of the microfluidic channel 102. Inanother embodiment, the actuation elements 136 may comprise at least onePeltier device for providing heating and/or cooling functionality. Instill further embodiments, the actuation elements 136 may compriseeither optical or microwave devices for providing heating and/or coolingfunctionality.

The microfluidic channel 102 may be represented by a plurality ofsectors 138 (138 a . . . 138 n), where each sector 138 of themicrofluidic channel 102 is positioned adjacent to and operativelyinterfaces with a respective one of the TCEs 136. In one embodiment, theTCEs 136 may be fabricated as a thermoelectric device with a series ofseparately-controllable parallel cooling elements. To operate themicrofluidic device 100, the thermoelectric actuation module 130 wouldhave sectors that are each 300-500 microns wide and 25 mm long. In analternative embodiment, the actuation module 130 may be fabricated as asingle thermoelectric module (e.g., 5×25 mm) that is attached to aseries of electrical resistive heaters in a pattern of parallel stripes.

In one mode of operation in one embodiment, the array of TCEs 136 arecontrolled such that each successive TCE 136 (e.g., starting from TCE136 a and moving toward TCE 136 n for n number TCEs 136, as illustratedin FIG. 4) would be switched in series from a warm state to a cold stateuntil a sufficient number of TCEs 136 are cold to induce the desiredfrozen portion 118 in the microfluidic sample 104, as illustrated inFIG. 1D. In the embodiment illustrated in FIG. 1H, at least one TCE 136n adjacent to the upper edge 116 of the microfluidic channel 102 can beused as necessary to freeze the portion 126 of the microfluidic sample104 while the melted pure solvent 128 is pumped out of the microfluidicchannel 102 into the adjacent second outlet 108 b.

Operating a freeze separation device and process on the microfluidicscale offers several advantages that are not available to macroscopicdevices: precise temperature control, repeated cycling of themicrofluidic sample 104 for fractional exclusion of the constituentelement(s) 120, using multiple simultaneous cycles for shortening theseparation process, the use of cross flow for improving separationperformance, utilizing geometries of the inlet 106, the microfluidicchannel 102 and the outlets 108 that enhance transportation of thefluids during the separation process, differential separation ofdifferent constituent elements 120 from the microfluidic sample 104, andthe mixing of fluids on a microfluidic scale. Each of these advantageswill be described separately below.

Precise Temperature Control

In one aspect, the growth rate of the ice front of the frozen portion118 of the microfluidic sample 104 is controlled by the temperaturegradient in the microfluidic device 100. If this temperature gradientgets too steep, the growth process can become unstable, leading toincomplete separation. Some of this instability is due to the unsteadynature of heat transport in systems large enough to have significantconvective heat transport. However, in the microfluidic scale such as inthe microfluidic device 100, the short distances between the ice frontof the frozen portion 118 of the microfluidic sample 104 and the TCEs136, as well as the lack of significant convective heat transport, makeit possible to provide much more precise temperature control that is notachievable on the macrosopic level. As such, it is possible to createhigher average temperature gradients (without needing to be concernedabout local thermal instabilities), leading to higher ice-front growthrates. In the embodiment of the microfluidic device 100 shown in FIGS. 2and 3 with independent control of each of the TCEs 136, it would bepossible to control the temperature of the microfluidic sample 104 oververy narrow sectors 138, and provide precise control on the growth rateof the ice front of the frozen portion 118 across the sectors 138.

Cycling

Separation induced by freezing on the macroscopic scale is imperfectbased on two principal issues. First, a small fraction of the solute canbe trapped in a growing ice front and, second, the ice front tends tojump over or around regions of high solute concentration, trapping themas described above. On the macroscopic scale, the only ways to preventthese problems are to slow the growth rate of the ice front and toinduce circulation in the fluid to dissipate temperature andconcentration gradients. Further, repeated cycling is difficult toperform on the macroscopic scale because convective transport re-mixesthe fluid as soon as it is melted.

In accordance with one feature of the disclosure, simpler methods ofrepeated cycling are made possible on the microfluidic scale for threereasons: 1) the lack of convective currents, 2) the presence of laminarflow, and 3) the small separation distances required. By utilizing themicrofluidic device 100 that takes advantage of these conditions ofmicrofluidic flow, if there is no significant back flow of thesolutes/particulates 120 being excluded, then the degree of purificationof the solvent 124 and concentration of the solutes/particulates 120 canbe increased through repeated cycling.

In one embodiment, the size of the frozen portion 118 or zone of themicrofluidic sample 104 is alternately increased and decreased bychanging the temperature profile across the microfluidic device 100. Asthe frozen zone 118 increases in size, the advancing liquid-solid phaseboundary tends to push solute molecules and/or particulates ahead of it.When the frozen zone 118 decreases in size, the retreating liquid-solidphase boundary does not drag solute molecules or particulates with it,but merely allows them to travel back into the previously frozen region.In microfluidic devices, there are no convection currents, so the backflow of solute molecules or particulates occurs only through diffusion,which is a relatively slow process. Thus, it is possible to increase thedegree of exclusion of solutes and particulates from the frozen zone 118by cyclically varying its size. In another embodiment, the size of thefrozen zone 118 is cycled around a steadily increasing mean, as shown bythe graphical illustration of FIG. 13. In this embodiment, each increasein size pushes the majority of solutes or particulates to the momentarymaximum of the frozen zone 118. Each swing back to a smaller size picksup another fraction of the remaining solutes or particulates to bepushed out during the next size increase. When the frozen zone 118 sizefollows the pattern illustrated in FIG. 13, each location in themicrofluidic device 100 experiences several freeze events. For examplethe location at 300 microns from the origin first freezes at fiveseconds, then melts and freezes six more times in the next ten seconds.By adjusting the amplitude and period of the oscillations relative tothe rate of increase of the mean, it is possible to control the numberof freeze events at any location.

Back flow of the excluded solutes/particulates 120 can be prevented bycontrolling the TCEs 136 a-136 n to sequentially freeze adjacent zonesof the microfluidic sample 104 in sequential sectors 138 a to 138 n ofthe microfluidic channel 102. Then, TCE 136 a may be actuated to re-meltthe frozen portion 118 in the first sector 138 a of the microfluidicchannel 102, followed by a sequence of steps in which the parallel arrayof TCEs 136 is controlled such that, periodically, for each TCE 136(x)for x=a,n that is warm, the next higher TCE 136(x+1) in the sequence isswitched from cold to warm while the TCE 136(x) is switched from warm tocold. The time lag between switching the two TCEs 136(x) and 136(x+1)can be either zero, or negative or positive, to account for thermal lagsin the system, or to take advantage of density changes associated withphase changes to induce pumping action (as described below). After aperiod T, the TCE 136(x+2) in the sequence will be switched from cold towarm while the TCE 136(x+1) will be switched back from warm to cold.

By repeating this process across all the TCEs 136 a-136 n, a zone ofmelting moves across the microfluidic channel 102 in a directionperpendicular to the direction of fluid flow through the microfluidicchannel 102 from the inlet 106 to the outlets 108. When the processreaches TCE 136 n, the next step is to warm TCE 136 a as TCE 136 n iscooled, thereby starting a new melt zone in sector 138 a of themicrofluidic channel 102. By repeating this cycle multiple times, thedegree of separation of the constituent element(s) 120 from themicrofluidic sample 104 through repeated fractional exclusion will beimproved relative to that which would be obtained from the fractionalexclusion of a single cycle. At the completion of the desired number ofcycles, the inlet valve 110 is re-opened, and the concentrated andpurified fluids are pumped out of the microfluidic channel 102 asdescribed herein. Any solutes or particulates in the microfluidic sample104 will be separated and concentrated in the upper sector 122 of themicrofluidic channel 102 to be removed through outlet 108 a while theremaining purified solvent 124, depleted of solute or particulates, willflow out through the lower outlet 108 b.

Multiple Simultaneous Cycles

In one embodiment, the time required for the separation process can beshortened by using multiple simultaneous separation cycles. As the zoneof melting moves across the microfluidic channel 102, it is possible tostart a new melt zone in sector 138 a before the previous melt zonereaches sector 138 n. It is only necessary to delay subsequent cyclessufficiently that there is an unbroken frozen portion 118 betweenadjacent melt zones. For example, when the first cycle gets to the pointwhere TCE 136 c is warm and TCE 136 b is cold, it would be possible toswitch TCE 136 a from cold to warm. In contrast, it would not bepossible to switch TCE 136 a to warm at the same time that TCE 136 b isswitched to cold because the frozen portion 118 in sector 138 a mustremain solid until the fluid in sector 138 b has solidified sufficientlyto prevent back flow from sector 138 c to sector 138 a. Thus, TCE 136 acould be switched to warm at the same time as TCE 136 d (a+3), TCE 136 g(a+6), and TOE 136 j (a+9), allowing four melt zones to traverse themicrofluidic sample 104 simultaneously. In one aspect, it is notnecessary that the melt zones in the microfluidic sample 104 be exactlyin phase with one another. For instance, TCE 136 a can be switched towarm as soon as sector 138 b is sufficiently cold to ensure that therecan be substantially no backflow, which may be before or after TCE 136 dis scheduled to switch states. Upon completion of the required number ofcycles, the concentrated and purified fluids are extracted from themicrofluidic channel 102 as indicated herein.

By melting portions of the microfluidic sample 104 in different sectors138 within the microfluidic channel 102 at different times, movement ofa liquid zone and/or a frozen zone of finite sizes can be accomplishedin a desired direction in the microfluidic sample 104. The array of TCEs136 can be controlled so that each successive TCE 136 is activated inseries to a hot state, with a time interval t₁ between activation of twosuccessive TCEs 136. Each TCE 136 can then be activated back to a coldstate at a time interval t₂ after it is activated to a hot state,wherein the time interval t₁ controls the speed at which a liquid zonemoves across the sectors and the time interval t₂ controls the width ofthe liquid zone.

Cross Flow

In another embodiment, the cycling embodiments described above mayfurther incorporate a cross flow through the microfluidic channel 102.In steady-state operation, the inlet valve 110 is left open, and themicrofluidic sample 104 to be separated enters continuously through theinlet 106. The TCEs 136 are controlled in the manner described above forcycling or multiple simultaneous cycling. The continuous flow of fluidat the inlet 106 causes a continuous flow of fluid at the outlet 108. Aslong as the freeze separation cycling rate employed by the microfluidicdevice 100 is fast enough relative to the flow rate of the microfluidicsample 104 through the microfluidic channel 102, the fluid willexperience a useful number of phase changes (e.g., freeze-thaw cycles)before leaving the microfluidic channel 102 to perform the requisitefractional exclusion of the constituent element(s) 120 from themicrofluidic sample 104.

Multiple Simultaneous Cycles

In another embodiment, as described above for the discrete separationprocess, it is possible to perform multiple simultaneous freezeseparation cycles across the microfluidic channel 102 as long as acontinuous frozen portion 118 is maintained between successive meltzones 124. In a cross-flow embodiment of the microfluidic device 100,the multiple cycles would preferably be separated from one another by adistance no greater than the width of the inlet 106.

Inlet Channel Modifications

In one embodiment, the inlet 106 may comprise a channel that need onlybe wide enough that there is always a path for fluid to flow.Specifically, when the device is operating with multiple simultaneousfreeze separation cycles, it will always be the case that at least oneof the sectors 138 a-138 d will be open (i.e., the zone of microfluidicsample 104 in at least one of the sectors 138 a-138 d will not befrozen). Thus, in one embodiment, the inlet 106 is preferably only thewidth of the first four sectors 138 a-138 d, as shown in FIG. 6. In thisconfiguration, no fluid will directly enter the remaining sectors 138e-138 n from the inlet 106. Instead, fluid will be transported acrossthe sectors 138 due to the expansion of the solvent in the microfluidicsample 104 on freezing. An alternative embodiment for fluids thatcontract upon freezing require a different configuration as described infurther detail below.

Integral Pumping

In the embodiments of the microfluidic device 100 employing a strictlyrectangular microfluidic channel 102, fluid transport would be from theinlet 106 toward the outlet 108 through open (e.g. non-frozen ornon-solid) sectors 138 that connect to both the inlet 106 and the outlet108. Fluid would also be transported across all sectors 138 due toexpansion of the microfluidic sample 104 on freezing. For sectors 138that are directly connected to the inlet 106, the net fluid transport isalong a diagonal direction, with the angle determined by the relativemagnitudes of the flow mechanisms (controlled by inlet 106 flow rate,sector 138 width, and the cycling frequency). For those sectors 138without a direct connection to the inlet 106, the net flow isperpendicular to the sector 138 boundaries based strictly on theexpansion of the microfluidic sample 104 on freezing. As such, with astrictly rectangular microfluidic channel 102, the fluid will tend tobuild up against the upper edge 16 of the microfluidic channel 102 insector 138 n, and no purified solvent 124 will be expelled to the outlet108 from those sectors 138 that are not in direct connection with theinlet 106. Structural issues in such an arrangement can be prevented byleaving a liquid zone above the last ice zone in the microfluidic sample104 while no further net separation occurs in sectors 138 that are notexpelling purified solvent 124 to the outlet 108.

In one embodiment, the shape of the microfluidic channel 102 can bemodified, as shown in FIG. 7, such that the upper edge 116 of themicrofluidic channel 102 extends in a diagonal direction cutting acrossthe array of parallel TCEs 136. In this embodiment, the expansion onfreezing of the solvent in the microfluidic sample 104 will force theliquid in each of the melt zones 124 to move from the inlet 106 towardsthe outlets 108 a and 108 b (left to right in FIG. 7), thereby expellinga portion of the melted purified solvent 124 through the outlet 108 b asit moves across the sectors 138. In steady-state operation, solvent inthe device 100 of FIG. 7 would show a concentration gradient (insolutions or particulates) steadily decreasing from the right upper,wider portion of the microfluidic channel 102 to the left lower,narrower portion of the microfluidic channel 102. With proper balancingof the inlet 106 flow rate, cycle frequency, and shape of the upper edge116 of the microfluidic channel 102, the fluid exiting at the end ofeach sector 138 into the outlet 108 b will have experienced a sufficientnumber of freeze-thaw cycles to obtain the required degree ofpurification. The majority of the solute and particulates 120 will betransported across the sectors 138 by freezing, and will exit throughoutlet 108 a adjacent to the uppermost sectors 138 (n−1, n). Thegeometry of the outlet 108 a, 108 b channels would be configured so asto prevent back flow between the outlet channels 108 at the adjacentends of the sectors 138.

In alternative embodiment, a microfluidic device 100 is provided forexpelling a portion of the fluid at each sector 138 to the outlets 108in which the spacing between adjacent sectors 138 gradually decreases inthe progression through the sectors 138 a-138 n. In this embodiment, theadvancing ice front of the frozen portion 118 will move slightly fasterthan the receding melt front ahead of it, and the consequentlydecreasing volume of the melt zone will cause fluid to be expelled fromthe outlet 108. There are then two options for dealing with the finalsector 138 n adjacent to the upper edge 116 of the microfluidic channel102. The first option, as mentioned above, is to have a permanent liquidzone in the microfluidic sample 104 beyond the last sector 138 n. Thesecond option is to modify the TCE 136 n for the last sector such thatfreezing progresses from the inlet 106 toward the outlet 108, therebyexpelling the excess liquid into the outlet 108 from the microfluidicchannel 102. Other geometries of TCEs 136 and respective sectors 138 mayalso be useful that satisfy the requirements that the volume of the meltzone must decrease as it moves through successive sectors 138 in themicrofluidic channel 102 and that one end of the melt zone must be opento the outlet 108.

In an alternative embodiment, further benefits may also be obtained bycombining all of these modifications into a single device, including adiagonal upper edge 116 (or other non-rectangular shape), steadilydecreasing spacing of the TCEs 136 and sectors 138, and inlet-to-outletdirectional freezing of the microfluidic sample 104. With adequate powerfor thermal control, it is also possible to scale this cross-flowfreeze-separation embodiment to macroscopic devices (with dimensionslarger than 1 mm). Thus, the cross-flow freeze-separation principles andembodiments described herein can be equally applied to accomplish freezeseparation of a constituent element from a fluidic sample in a fluidicdevice of both microscopic and macroscopic dimensions.

Non Aqueous Solutions

A different configuration is required for dealing with fluids that(unlike water) contract on freezing and expand on melting. Withwater-based solvents, the freeze-induced lateral transport of thesolvent is in the direction of the advancing ice front. The lateralspeed of fluid motion in each melt zone is proportional to the speed ofthe ice front, and the proportionality constant is the difference in theliquid and solid densities divided by the liquid density. Thus,V_(L)=V_(S)(ρ_(L)−ρ_(S))/ρ_(L), where V_(L) is the speed of liquidtransport in the direction perpendicular to the ice front, V_(S) is thespeed of the advancing ice front, ρ_(L) is the density of the liquid,and ρ_(S) the density of the solid. In water, ρ_(S) is smaller thanρ_(L), so V_(L) will have the same sign as V_(S). The same equationapplies if the solid has a higher density than the liquid (contracts onfreezing), in which case, V_(L) and V_(S) will have opposite signs. Assuch, the net fluid transport in non-aqueous solutions is in thedirection opposite the direction of the advancing solidification front.Thus, an alternative embodiment of a microfluidic device 200 is providedfor fluids that contract on freezing, as illustrated in FIG. 8, with thefluid inlet 202 adjacent to the upper edge 204 of the microfluidicchamber 102, a diagonally extending lower edge 206 of the microfluidicchamber 102, and an array of TCEs 136. The TCEs 136 are controlled suchthat the melt zones in the microfluidic sample 104 traverse the TCEsectors 136 from the upper edge 204 of the microfluidic chamber 102 toits lower edge 206. The diagonal lower edge 206 of the microfluidicchamber 102, combined with freeze-induced transport of fluid against thedirection of travel of the melt zones, will cause fluid to be pumpedtoward the outlets 108 a and 108 b.

Similarly to the case with water, fluid transport with non-aqueoussolutions can also be induced in a rectangular microfluidic chamber 102by varying the width of the melt zone across the sectors 136, In thiscase, however, it would be necessary to have the sectors 136 increase inwidth as the sectors move from the upper edge 204 toward the lower edgeof the microfluidic chamber 102. In this configuration, an expandingsolidification front would be moving slightly slower than theliquefaction front it is chasing. The volume of liquid in the melt zonewould thus be growing faster than the volume of the frozen zone, soliquid would be expelled. In general, any configuration that providesfor a melt zone that increases in volume over successive sectors 136will cause pumping in a fluid that expands on melting.

Differential Separation

In an alternative embodiment, different constituent elements 120 can beexcluded by the freeze separation process performed on the microfluidicsample 104 by capitalizing on the different degrees in which anadvancing ice front excludes different constituent elements 120. In thismanner, the freeze separation process thus may be useful for separatingdifferent solutes from one another. Referring now to FIG. 9,microfluidic device 300 is illustrated with a set of three outlets 108a, 108 b, 108 c connected to the microfluidic channel 102. Solutes thatare strongly excluded from the frozen portion 118 (ice phase) of themicrofluidic sample 104 during the freeze separation process areconcentrated toward an upper sector 302 of the microfluidic channel 102adjacent to the top outlet 108 a. Solutes that are less stronglyexcluded from the frozen portion 118 (ice phase) of the microfluidicsample 104 during the freeze separation process are concentrated towarda central sector 304 of the microfluidic channel 102 adjacent to thecentral outlet 108 b. The remaining purified solvent 124 is concentratedin the bottom sector 306 of the microfluidic channel 102 adjacent to thelower outlet 108 c.

Mixing

On the microfluidic scale, mixing has always presented problems becauseflow tends to be strictly laminar and diffusion has conventionally beentoo slow to be useful. In an alternative embodiment, a microfluidicversion of zone leveling is provided to overcome the prior problems withmixing fluids on a microfluidic scale. Referring to FIG. 10, amicrofluidic mixing device 400 is illustrated having a first inlet 402and a second inlet 404 for respectively introducing a first fluid and asecond fluid into a microfluidic channel 406. An array oflinearly-spaced parallel TCEs are positioned adjacent to respectivesectors 408 in the microfluidic channel 406 for inducing phase changesin respective zones of the first and second fluids located in themicrofluidic channel 406. The first and second fluids flow into themicrofluidic mixing device 400 while a series of solidification frontsmove across the sector 408 in alternate directions to mix the first andsecond fluids together. An outlet 410 is connected to the microfluidicchannel 406, where the fluid flowing out of the microfluidic channel 406is a relatively uniform mix of the first and second fluids introducedinto the microfluidic channel 406. In an alternative embodiment, themicrofluidic mixing device 400 can be run in a batch mode by valving theinlets 402, 404 and/or the outlet 410. In another alternativeembodiment, an output fluid can be produced with a concentrationgradient of solute across the width of the output 410 channel bycontrolling the cycling frequency relative to the flow speed of thefluids in order to produce an output fluid that is only partially mixed.Concentration gradients could also be established by having sequentialice fronts move only part way across the microfluidic channel 406, or byhaving more ice fronts moving in one direction than in the other.

In another embodiment, the microfluidic mixing device 500 can include acircular or ring-shaped microfluidic channel 502, as shown in FIG. 11,having a melt zone moving continuously in either direction around thering-shaped microfluidic channel 502. There are again two inlets 504 a,504 b for introducing first and second fluids and a single outlet 506for outputting the mixed fluids. A plurality of TCEs 508 are arranged ina radial pattern adjacent to the ring-shaped microfluidic channel 502,such that the ice front and melt zone move in a circular direction 510around the ring-shaped microfluidic channel 502. In one embodiment, themicrofluidic mixing device 500 could operate with multiple simultaneousmelt zones and ice fronts, as described above with respect to otherembodiments. In another embodiment, it is possible to mix more than twofluids by having a plurality of inlets 504 a, 504 b, 504 c, 504 d, etc.,as illustrated in FIG. 12, where an inlet 504 can be provided for eachadditional fluid to be introduced into the microfluidic channel 502 formixing. In one aspect, each of the inlets 504 a-504 d and the outlet 506possess a width such that each would always be connected to at least onemelt zone in the mixed fluids, so that flow would be continuous betweenthe inlets 504 a-504 d and the outlet 506. In one embodiment, the fluidsin the central portion 512 of the ring-shaped microfluidic channel 502are kept in a liquid state to allow for flow through. The device is alsoversatile in the sense that any of the inlets 504 a-504 d and the outlet506 can serve as flow channels to be either an inlet or outlet. Inanother embodiment, the inlets 504 a-504 d and the outlet 506 can bevalved so that the device 500 can be operated in a batch mode.

What is claimed is:
 1. A method for separating a constituent elementfrom a fluidic sample, the method comprising: providing a fluidic devicecomprising a fluidic channel and an actuation module including Peltierdevices operatively interfaced with the fluidic channel, the fluidicchannel having a length and a width; introducing a portion of a fluidicsample into the fluidic channel in the fluidic device; by the Peltierdevices, freezing different sectors of the portion of the fluidic sampleacross the width of the fluidic channel at different times than oneanother, the freezing of the different sectors at the different timescausing fractional exclusion of a constituent element from at least oneof the sectors and concentration of the fractionally excludedconstituent element within at least one other of the sectors; removingthe fractionally excluded constituent element from the fluidic channel,wherein the remaining sectors of the fluidic sample from which theconstituent element has been fractionally excluded include a purifiedfluidic sample; and removing the purified fluidic sample from thefluidic channel such that the removed purified fluidic sample remainssubstantially separated from the removed fractionally excludedconstituent element.
 2. The method of claim 1, wherein the constituentelement is fractionally excluded from a frozen sector of the portion ofthe fluidic sample.
 3. The method of claim 1, wherein the constituentelement is fractionally excluded from a liquid sector of the portion ofthe fluidic sample.
 4. The method of claim 1, wherein the freezingdifferent sectors of the portion of the fluidic sample at differenttimes than one another is performed using respective ones of the Peltierdevices in order to cause movement of the constituent element across thewidth of the fluidic channel.
 5. The method of claim 4, wherein said therespective ones of the Peltier devices are actuated in a sequentialorder.
 6. The method of claim 1, further comprising heating the fluidicsample to thaw the frozen sectors of the portion of the fluidic sample,wherein the separated constituent element remains substantiallyseparated from the thawed sectors of the portion of the fluidic sample.7. The method of claim 1, wherein the Peltier devices are arranged in anarray elements situated adjacent to the fluidic channel and configuredto freeze and heat respective ones of the sectors.
 8. The method ofclaim 7, wherein the Peltier devices are arranged in an array aligned ina parallel manner with one another across the width of the fluidicchannel.
 9. The method of claim 8, further comprising controlling thePeltier devices so that each successive Peltier device is activated inseries to a cold state to freeze a sector of the portion of the fluidicsample adjacent to the respectively activated Peltier device.
 10. Themethod of claim 1, wherein some of the Peltier devices maintain thesample in a predominantly frozen state, the method further comprisingmelting frozen sectors of the portion of the fluidic sample at differenttimes than one another using respective ones of the Peltier devices inorder to cause movement across the width of the fluidic channel of theconstituent element being separated from the fluidic sample.
 11. Themethod of claim 1, wherein the fluidic sample is a solution including asolute and a solvent, further wherein the solute is the constituentelement that is separated from the fluidic sample.
 12. The method ofclaim 1, wherein the fluidic sample includes a liquid and suspendedparticulates, further wherein the suspended particulates comprise theconstituent element that is separated from the fluidic sample.
 13. Themethod of claim 1, wherein the separated constituent element is removedfrom the fluidic channel through a first outlet in fluidic communicationwith the fluidic channel.
 14. The method of claim 13, wherein thepurified fluidic sample is removed from the fluidic channel through asecond outlet in fluidic communication with the fluidic channel andstructurally distinct from the first outlet.
 15. The method of claim 1,wherein the fluidic channel includes an inlet for introducing theportion of the fluidic sample into the fluidic channel, the inlet havinga width less than the maximum width of a frozen zone of the fluidicsample, the method further comprising introducing the portion of thefluidic sample into the fluidic channel through the inlet.
 16. Themethod of claim 1, further comprising maintaining a cross flow of thefluidic sample through the fluidic channel during separation.
 17. Themethod of claim 1, wherein the fractionally excluded constituent elementcomprises a contaminant.
 18. The method of claim 1, wherein the fluidicchannel includes an inlet and at least one outlet where the fluidicchannel increases in size as it extends from the inlet to the at leastone outlet, the method further comprising integrally pumping theseparated constituent element and fluidic sample toward the at least oneoutlet utilizing the increase in size of the fluidic channel whenfreezing the different sectors of the portion of the fluidic sample. 19.The method of claim 1, wherein the fluidic channel includes an inlet andat least one outlet where the fluidic channel decreases in size as itextends from the inlet to the at least one outlet, the method furthercomprising integrally pumping the separated constituent element andfluidic sample toward the at least one outlet utilizing the decrease insize of the fluidic channel when freezing different sectors of theportion of the fluidic sample.
 20. The method of claim 1, wherein thefluidic sample is a solution containing at least two solutes, the methodfurther comprising performing repeated cycles of freezing and heating ofdifferent sectors of the portion of the fluidic sample with respectiveones of the Peltier devices to create a plurality of layers ofconstituent elements across the width of the fluidic channel, whereineach layer of the plurality includes predominantly a differentconstituent element than each other layer of the plurality.
 21. Themethod of claim 20, wherein the fluidic channel includes an inlet and aplurality of outlets, the method further comprising pumping each layerof the plurality of layers of constituent elements to a respective oneof the plurality of outlets.
 22. The method of claim 1, wherein at leastone sector of the fluidic channel adjacent to a first one of the Peltierdevices contains a different concentration of the excluded constituentelement than a concentration of the excluded constituent element inanother sector of the fluidic channel adjacent to a second one of thePeltier devices.
 23. The method of claim 1, further comprising:alternately increasing and decreasing a number of the frozen differentsectors of the portion of the fluidic sample using the Peltier devicesto cause additional fractional exclusion of the constituent element fromthe fluidic sample.